fix(subscriptions): migrated subscription switch

[miguelpeixe]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

NPPM-2434

Migrated subscriptions don't have an initial order, which is required by Woo Subscription to allow switches. This is checked via the last_order_date_created meta.

This PR implements a strategy to backfill this meta using the scheduled start date value. This should happen on demand while generating the switch cart.

How to test the changes in this Pull Request:

1. Make sure you have a variable subscription or a grouped product as your primary subscription product (Audience -> Subscriptions)
2. Manually create a subscription for a reader via wp-admin (WooCommerce -> Subscriptions -> Add Subscription)
3. Make sure to add a line item for a variant of the primary subscription product
4. In a new session, visit the site with ?upgrade-subscription=1 and sign in as the reader
5. Confirm you are not able to continue checkout, as the strategy is only meant for migrated subscriptions
6. Manually set a _piano_subscription_id or _stripe_subscription_id order meta with any value to the subscription
7. Visit the ?upgrade-subscription=1 again and confirm you are able to complete the switch

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[Copilot]

Pull request overview

This PR fixes subscription switching for migrated subscriptions that lack an initial order. WooCommerce Subscriptions requires the last_order_date_created meta to allow subscription switches, but subscriptions migrated from Piano or Stripe don't have this value.

Key Changes:

• Implements a filter hook to detect migrated subscriptions without a last order date
• Backfills the last_order_date_created meta using the subscription's scheduled start date
• Performs validation checks to ensure the subscription can be switched before allowing the operation

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[matticbot]

🎉 This PR is included in version 6.27.1 🎉

The release is available on GitHub release

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